Reducing oxygen diffusion resistance in the cathode catalyst layer of polymer electrolyte fuel cells under humidified conditions is essential for reducing the cost of expensive platinum catalysts. We designed an ionomer swelling model that reproduces isotherm adsorption and oxygen diffusion coefficient measurements, to elucidate the effect of pore blockage due to ionomer swelling and capillary condensation on the decrease in oxygen diffusivity. Coupled simulations of ionomer swelling, capillary condensation, and oxygen diffusion are performed using this model, and the water and swelled ionomer distributions at each relative humidity are elucidated. When the humidity is low, ionomer swelling is dominant for the oxygen diffusivity reduction, whereas when the humidity is high, both ionomer swelling and capillary condensation affect them.